1. Field of the Invention
This invention relates to a method of detecting cylinder pressure in internal combustion engines and more particularly to a method of detecting cylinder pressure wherein the crankshaft angle at which the maximum pressure occurs is detected through approximation.
2. Description of the Prior Art
Recent years have been increasing adoption of internal combustion ignition timing control methods in which the maximum cylinder pressures (Pmax) in the respective power strokes are detected and the ignition timing is controlled so that the maximum cylinder pressure angles .theta.pmax, i.e. the crankshaft angles at which Pmax occurs, converge toward a predetermined crankshaft angle. A conventional method used for this purpose is disclosed in Japanese Patent Publication No. 58(1983)-33394. In this prior art method, the output of a pressure sensor is differentiated using a differentiation circuit and the result of the differentiation is applied to a comparison circuit for comparison with a predetermined value. However, as this method relies on analog circuitry for the detection, it is not totally satisfactory with regard to accuracy or with regard to tracking performance during a high-speed engine operation. Another method of this type, described in Japanese Unexamined Patent Publication No. 57(1982)-173565, relates to a method of using an A/D converter to A/D convert the output of a cylinder pressure sensor once per prescribed angle of crankshaft rotation and defining the crankshaft angle at the time the converted value reaches a maximum as the maximum cylinder pressure angle .theta.pmax. In order to realize good detection accuracy with this second prior art method, however, it has been necessary to obtain a large number of pressure value samples and this has required use of high-speed A/D conversion. Thus for practical application of the method it has been necessary to use a high-speed A/D converter and a large capacity memory. This method is therefore disadvantageous in that it requires expensive equipment to carry out.